1. Field of the Invention
The present invention relates to methods and systems for high-speed, high-resolution 3-D imaging of manufactured parts of various sizes, such as threaded fasteners and cartridge cases.
2. Background Art
Inspection of defects on small arms ammunition cartridges and cases is a vital aspect in the manufacturing process, allowing for maintenance of a high level of quality and reliability in the munitions industry. Standards have been developed and applied by manufacturers for many years to assist in classifying various types of defects. Alternatively, a military standard is used such as that introduced in 1958 by the US Department of Defense, MIL-STD-636. For small arms ammunition calibers up to .50, this standard serves to evaluate and illustrate a practical majority of defects assembled as a result of extensive surveys covering all the small arms ammunition manufacturing facilities in the United States.
FIGS. 1a and 1b are side and bottom schematic views, respectively, of a .50 caliber case. As explained in the above-noted military standard, a case is to be counted as a defective because of a split case if the cartridge case shows a definite separation of the metal entirely through the case wall. A case is to be classified as either a “major” or “critical” defect depending on the location of split. A split in the neck (I), taper (S) or case (J) position shall be counted as a “major” defect when no loss of powder occurs; and as a “critical” defect when loss of powder occurs. A split in the case (K), groove (L) or head (M) position shall be counted as a “critical” defect.
FIGS. 1c and 1d are side and bottom schematic views, respectively, of a .30 caliber case. As noted above, a case is to be counted as a defective because of a split case if the cartridge case shows a definite separation of the metal entirely through the case wall. A case is to be classified either as a “major” or “critical” defective depending on location of split. A split in the (I) or (J) position shall be counted as a “major” defect when no loss of powder occurs; and as a “critical” defect when loss of powder occurs. A split in the (K), (L) or (M) position shall be counted as a “critical”defect.
FIGS. 1e and 1f are side and bottom schematic views, respectively, of a .45 caliber case. Again, as noted above, a case is to be counted as defective because of a split case if the cartridge case shows a definite separation of the metal entirely through the case wall. A case is to be classified either as a “major” or “critical” defective depending on the location of the split. A split in the (I) or (J) position shall be counted as a “major” defect when no loss of powder occurs. A split in the (K), (L) or (M) position shall be counted as a “critical” defect.
U.S. Pat. No. 4,923,066 discloses an automatic visual inspection system for small arms ammunition which sorts visual surface flaws at high speed according to established standards. The system comprises interface apparatus for receiving a supply of ammunition cartridges and providing each cartridge with a predetermined orientation, conveying apparatus for locating each of the cartridges for inspection in at least one inspection station, apparatus for imaging selected areas of each cartridge to provide video surface feature data associated therewith, and apparatus for processing the video surface feature data to detect the presence of a predetermined set of characteristics and provide output signals in accordance therewith, the conveying apparatus being operated to sort each of the inspected cartridges in accordance with the output signals. Since many surface flaws look the same in two dimensions, such as scratches and splits or acid holes and stains, special lighting of the cartridges is used so that discrimination between them can be achieved on the basis of off-specular reflections.
U.S. Pat. No. 7,403,872 discloses a method and system for inspecting manufactured parts, such as cartridges and cartridge cases, at a plurality of inspection stations including a circumference vision station and primer and mouth vision stations.
PCT Patent Application No. WO 2005/022076 A2 discloses a part inspection apparatus including two embodiments of a self-centering clamp which drops parts to be inspected.
As described in U.S. Pat. No. 6,098,031, triangulation is the most commonly used 3-D imaging method and offers a good figure of merit for resolution and speed. U.S. Pat. Nos. 5,024,529 and 5,546,189 describe the use of triangulation-based systems for inspection of many industrial parts, including shiny surfaces like pins of a grid array. U.S. Pat. No. 5,617,209 shows a scanning method for grid arrays which has additional benefits for improving accuracy. The method of using an angled beam of radiant energy can be used for triangulation, confocal or general line scan systems. Unfortunately, triangulation systems are not immune to fundamental limitations like occlusion and sensitivity to background reflection. Furthermore, at high magnification, the depth of focus can limit performance of systems, particularly edge location accuracy, when the object has substantial relief and a wide dynamic range (i.e. variation in surface reflectance). In some cases, camera-based systems have been combined with triangulation systems to enhance measurement capability.
Confocal imaging, as originally disclosed by Minsky in U.S. Pat. No. 3,013,467, is similar to computerized tomography where slices in depth are sequentially acquired and the data is used to “reconstruct” a light scattering volume. In principle, an image is always formed of an object at a focal plane as taught in elementary physics, but over a region of depth there are an infinite number of planes which are out of focus yet return energy. That is to say that the lens equation for image formation is based on an idealization of an “object plane” and “image plane.”
In the case of conventional confocal imaging, the slices are determined from the in-focus plane, and out-of-focus light (in front and back of the focal plane) is strongly attenuated with a pinhole or slit. Typical confocal systems use fine increments for axial positioning for best discrimination between adjacent layers in depth, for example, semi-transparent biological samples. However, the method need not be restricted to the traditional transparent or translucent objects, but can be applied both as a depth measurement tool and image enhancement method using reflected light for contrast improvement through stray light rejection. As with any method, there are advantages and disadvantages.
U.S. Pat. No. 5,098,031 discloses a versatile method and system for high-speed, 3-D imaging of microscopic targets. The system includes confocal and triangulation-based scanners or subsystems which provide data which is both acquired and processed under the control of a control algorithm to obtain information such as dimensional information about the microscopic targets which may be “non-cooperative.” The “non-cooperative” targets are illuminated with a scanning beam of electromagnetic radiation such as laser light incident from a first direction. A confocal detector of the electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation. The triangulation-based subsystem also includes a detector of electromagnetic radiation which is placed at a second location which is non-collinear with respect to the incident beam. Digital data is derived from signals produced by the detectors.
U.S. Pat. No. 5,815,275 discloses triangulation-based 3-D imaging using an angled scanning beam of radiant energy.
Published U.S. Patent Applications 2009/0103107 and 2009/0103112 disclose part inspection using a profile inspection subsystem and triangulation.
U.S. Pat. Nos. 2,444,457; 4,778,252 and 4,938,489 disclose self-centering holders for optical elements.
Other U.S. patents related to the invention include U.S. Pat. Nos. 4,547,674 and 4,970,401.